The present invention relates to the art of semiconductor packaging, and more particularly to methods and apparatus for testing the bonding of surfaces in the art of packaging semiconductor devices.
Semiconductor chips or dies are packaged to tie the circuits thereof to the outside world. The chip is normally bonded to a substrate, such as ceramic in the process of packaging. Adhesive pastes have been used to secure heat sinks to ceramic semiconductor packages. In addition, metallic-glass pastes have been used for attaching semiconductor chips to ceramic substrates in the manufacture of semiconductor devices, and particularly, large scale integrated (LSI) and very large scale (VLSI) circuits.
These circuits typically include a semiconductor chip or die, e.g., silicon, gallium arsenide, etc., which is bonded to a supporting ceramic substrate. Commonly assigned U.S. Pat. Nos. 4,636,254, 4,761,224 and 4,968,738 are directed to improved silver-glass die attach pastes for the attachment of a semiconductor die to a ceramic substrate. These patents disclose silver-glass die attach pastes consisting generally of a mixture of silver flake and glass frit distributed in an organic vehicle, including a suitable organic resin and a suitable organic solvent.
To form a bond between a semiconductor die and a ceramic wafer or substrate, a selected quantity of the silver-glass paste is placed between the opposing die and substrate surfaces in a sandwiched relationship. The sample is dried and fired in a oven or furnace at temperatures above the glass transition temperature (T.sub.g) of the glass constituent. During the drying and firing stages, the organic vehicle volatilizes and the glass flows to wet the ceramic substrate and die, while the silver flake sinters together. Upon cooling, the result is a secure bond between the die and the substrate.
One critical aspect of the semiconductor die/ceramic substrate bonding process is that the post-fired adhesive film must have an adequate bond strength to accommodate differential thermal expansion. If the bond line thickness is sufficient, the resultant bond will exhibit good resistance to differential thermal expansion rates between the die and substrate and will produce a bond having a high tensile strength. Interfacial stress that arises from a thermal mismatch between the die and substrate is directly proportional to both the area of attachment and the modulus of elasticity of the bonding adhesive. This stress is also inversely proportional to the bond line thickness. Since the modulus of elasticity for a given die attach adhesive is fixed, the only avenue available to reduce interfacial stress is to maintain a sufficient bond line thickness on each part assembled. For the proper bonding of silicon die to ceramic substrates, the bond line thickness must be increased proportionally to the area of the surfaces to be bonded.
A major aspect of the quality control of such packaging is adequate and reliable testing to insure uniform and adequate bond strength. The prior art method of testing is carried out by bonding studs to the face of the chip and to the face of the substrate by means of an epoxy that is cured in an oven. One common form of the studs have a configuration similar to that of a nail, with a flat head and with a blunt point. An extender plate is sometimes used between the head and chip where the chip is considerably larger than the stud head.
Another form of device is like a bolt with a threaded shank. The head of the studs are bonded to the chip and substrate. The shank of the studs are either screwed or clamped in tensile test stand and a force applied to separate the chip from the substrate. This is intended to give a measure of the strength of the bond.
A major problem with the prior art approach is that the studs may not be bonded precisely perpendicular to the surface of the chip and/or the studs may not be aligned. This results in a peel test rather than a true tensile test. Therefore, this method is not always accurate. For example, in some instances, it is found that the chip is peeled from substrate because of the lines of force. Thus, a true tensile strength is not measured.
Accordingly, it is desirable that an improved adhesion tensile strength testing apparatus and method be available.